[en] The activity sources in a reactor primary coolant are discussed and the neutron activity is calculated. In the experimental work a simple loop was constructed in a TRIGA MkII research reactor simulating the essential radioactivity features of a LWR primary circuit. With this loop the physical parameters of a failed fuel element detection system like location of foils, exposition time, etching conditions and temperature dependence was studied. It was demonstrated that delayed neutron detection with track-etch film detectors is feasible. From the various film materials investigated, the type Makrofol KG showed the best over-all properties taking into account the response to thermal neutron induced fission fragments and chemical, physical and mechanical properties. With visual track counting the neutron flux density should not exceed 5.10⁴cm^-2s^-1 and the fluence should be kept below 1,5x10⁸cm^-2. There is a linear dependence between track density and the thermal flux density with a standard deviation of +-15%. With automatic counting systems the flux density and fluence values could be increased and the standard deviation reduced to some +-5%. It is proposed that the track-etch film technique could be a back-up system for an independent delayed neutron control providing also a permanent record